Embodiments of the invention relate generally to electronic converters and, more particularly, to a system for optimizing switching dead-time in a multi-switch power converter.
FIG. 1 illustrates a known half-bridge circuit topology 2 of a fundamental building block for a power converter. Half-bridge circuit 2 includes a pair of switches T1, T2 that may be controlled in according to a pulse-width modulation (PWM) scheme to convert DC voltage (vdc) to an AC waveform on the voltage output (vout) to control an AC load such as an AC motor, for example. In another example, the half-bridge circuit 2 may be used for one phase of a single- or multi-phase DC-to-DC converter.
Typically, switches T1, T2 are operated in an alternating manner in which one switch is an on state while the other switch is in the off state. Controlling which switch is in the on state via the PWM scheme causes the AC waveform on the voltage output (vout) to be generated according to a desired frequency.
However, the switching of a switch (e.g., switch T1) from its on state to its off state is not an instantaneous process. That is, it takes some time for switch T1 to stop conducting current therethrough. If the other switch (e.g., switch T2) begins conducting current prior to the shut-off of current through switch T1, a “shoot-through” condition may be created in which the DC voltage (vdc) becomes shorted, possibly damaging the voltage source supplying vdc.
Accordingly, in the traditional implementation of half-bridge circuit 2, a dead-time is calculated and added to the PWM scheme to avoid activating both switches T1, T2 to their on states simultaneously. As shown in FIG. 2, a PWM scheme 4 includes a plurality of pulses 6 for controlling switch T1 between its on and off states and a plurality of pulses 8 for controlling switch T2 between its on and off states. A dead time 10 is inserted between adjacent pulses 6, 8 to avoid creating a shoot-through condition. In this PWM scheme, a multi-channel PWM signal generator having separate controls for respective switches is used to enable dead time generation between separate pulses 6, 8.
The addition of dead time 10 in PWM scheme 4, while avoiding shoot-through, can cause the AC load to function differently than expected as a function of the dead time 10. For example, the dead time may cause the actual rotor flux of an AC adjustable speed drive to have a phase shift from its estimated flux, and the actual motor speed of the adjustable speed drive may be harmonically distorted about the estimated motor speed. In addition, the dead time affects total harmonic distortion of the motor current of the adjustable speed drive.
It would therefore be desirable to have a system for optimizing the dead time to reduce or minimize effects of the dead time on load function.